Polyamines, a class of aliphatic amines, are active in the growth and development of bacteria, animals, and plants. To better understand their physiological role in plants, we used spermidine and the inhibitor of SAMDC to analyze the developmental patterns of roots and shoots from Arabidopsis seedlings and in-vitro carnation shoot cultures. We also monitored mRNA levels of the polyamine biosynthetic gene after adding various phytohormones to the growing media. Treating wild-type Arabidopsis seedlings with polyamine resulted in greater growth after four weeks; 1 mM spermidine increased root lengths by 39% and seedling weights by 44%. Spermidine was more effective in enhancing root growth in the det2 mutants than in the wild-type plants. In contrast MGBG, an irreversible inhibitor of SAMDC activity, resulted in root lengths that were 29% of the controls, as well as seedling weights that were only 77% of normal. In our carnation cultures, shoot-growth rates were severely reduced by both spermidine and MGBG; root growth was also markedly inhibited by treatment with spermidine. Transcripts of ADC and SAMDC were significantly greater in cultures treated with kinetin and IAA, with levels being higher with the former, Previous research has shown that sensitivity and the appropriate cellular content for polyamines vary among plant species, and may also be organ- or tissue-specific. Polyamines may play an important developmental role because of their potentially stimulatory effects on phytohormonal signaling and/or synergism.